The present disclosure relates generally to polymer production and, more specifically, to removing diluent from slurry discharged from a polymerization reactor.
This section is intended to introduce the reader to aspects of art that may be related to aspects of the present approaches, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the embodiments described herein. Accordingly, it should be understood that these statements are to be read in this light, and not to as admissions of prior art.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into longer chains (or polymers) have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various industrial products.
Specific types of polyolefins, such as high-density polyethylene (HDPE), have particular applications in the manufacture of blow-molded and injection-molded goods, such as food and beverage containers, film, and plastic pipe. Other types of polyolefins, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP) are also suited for similar applications. The mechanical requirements of the application, such as tensile strength and density, and/or the chemical requirements, such thermal stability, molecular weight, and chemical reactivity, typically determine what type of polyolefin is suitable.
One benefit of polyolefins is that they are generally non-reactive with goods or products which they may contact. This allows polyolefin products to be used in residential, commercial, and industrial contexts, such as food and beverage storage and transportation, consumer electronics, agriculture, shipping, vehicular construction and so forth. The wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
To satisfy this demand, various processes exist by which olefins may be polymerized to form polyolefins. Typically, these processes are performed at petrochemical facilities, which have ready access to short-chain olefin molecules such as ethylene, propylene, butene, pentene, hexene, octene, decene, and other building blocks of the much longer polyolefins. Such monomers and comonomers may be polymerized in a liquid-phase polymerization reactor and/or gas-phase polymerization reactor to form a product including polymer (polyolefin) solid particulates, which are typically referred to as polymer fluff or fluff. The fluff may possess one or more melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), modulus, and crystallinity. The reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, and so forth, may also be a factor in achieving the desired fluff properties.
In addition to the one or more olefin monomers, a catalyst for facilitating the polymerization of the monomers may be added to the reactor. For example, the catalyst may be a particle added via a reactor feed stream and, once added, suspended in a fluid medium (e.g., a diluent, a monomer, or both) within the reactor. One example of such a catalyst is a chromium oxide containing hexavalent chromium on a silica support. In some polymerization processes, a diluent may be introduced into the reactor. The diluent may be an inert hydrocarbon, such as isobutane, propane, n-pentane, i-pentane, neopentane, and n-hexane. Such diluents may be selected such that they are in the liquid phase under reactor conditions. However, some polymerization processes may not employ a separate diluent. For example, in some cases of polypropylene production, the propylene monomer may itself act as a diluent.
The high demand of polymers produced by processes such as these often require a large amount of polymer to be so produced in a relatively short amount of time. Accordingly, some reactors may operate on a substantially continuous basis, where the reactor receives a steady stream of polymerization components (e.g., monomer, diluent, catalyst) and has a concomitant steady discharge. For example, the discharge of the reactor typically includes the polymer fluff as well as non-polymer components, such as unreacted olefin monomer (and comonomer), diluent, and so forth. In the case of polyethylene production, the non-polymer components typically include a primary diluent, such as isobutane, having a small amount of unreacted ethylene (e.g., 5 wt. %). This discharge stream may be continually processed or to processed in large batches, such as by a diluent/monomer recovery system, to separate the non-polymer components from the polymer fluff. The recovered diluent, unreacted monomer, and other non-polymer components from the recovery system may be treated, such as by treatment beds and/or a fractionation system, and ultimately returned as purified or treated feed to the reactor. Some of the components may be flared or returned to the supplier, such as to an olefin manufacturing plant or petroleum refinery. As for the recovered polymer (solids), the polymer may be treated to deactivate residual catalyst, remove entrained hydrocarbons, dry the polymer, and pelletize the polymer in an extruder, before the polymer is sent to a customer.
The competitive business of polyolefin production continuously drives manufacturers to improve their processes in order to lower operating and capital costs. In an industry where billions of pounds of polyolefin product are produced per year, small incremental improvements, for example, in catalyst activity, monomer yield, energy efficiency, diluent recovery, and so forth, can generate significant cost savings in the manufacture of polyolefins. Accordingly, there is a need for increased efficiency in polymer production and treatment.